Emulsion polymerization is one of several methods for effecting the process of addition polymerization. In emulsion polymerization the monomer is emulsified in a medium (generally water) with the aid of an emulsifying agent, surfactant, or micelle generating substance (such as rosin, fatty acids, soaps, alkyl sulfonates, etc). In addition to the monomer, emulsifier, and water (as the continuous phase), the process also requires an initiator. Hydrogen peroxide and ammonium peroxysulfonate are examples of such initiators.
One of the principal advantages of emulsion polymerization is the possibility of formation of a polymer of high molecular weight at a very high rate of polymerization. In this respect emulsion polymerization stands alone among techniques of free radical polymerization. Thus, this process has gained widespread commercial acceptance.
In U.S. Pat. No. 2,300,056, Meis and Ludwig developed emulsifiers for emulsion polymerization from mixtures of (a) water soluble surface active emulsifying agents and (b) water soluble salts of unsaturated carboxylic acids, the esters of which represent drying oils (particularly alkali metal salts).
In commonly assigned U.S. Pat. No. 4,259,459, Force disclosed acid-catalyzed, formaldehyde-treated mixtures of tall oil fatty and resin acids as emulsion polymerization emulsifiers. Also, in Emulsion Polymerization, Theory and Practice (1975), at Chapter 7, Blackley discusses the use of various fatty acid soaps and rosin acid soaps as micelle generating substances. Of considerable concern, however, is the effect upon polymerization which may be caused by adventitious impurities in the fatty acid or rosin acid soap used, particularly soaps derived from unsaturated fatty acids. Wilson et al. reported in Industrial Engineering Chemistry (1948), Vol. 40, p. 530, that soaps derived from linoleic and linolenic acids retard the copolymerization of styrene and butadiene. Naturally, the emulsifier also should not disturb the physical properties of the polymer.
Another major concern for producers and consumers of polymer products is the migration of emulsifier from the polymer. In some cases the result is merely a visual or aesthetic problem, while in other cases the result can be more dramatic (as with hoses for gasoline and other fuels). For those end uses where migration of emulsifier is of concern, two approaches can be taken to minimize problems: (1) washing the emulsifier from the latex before further converting operations or (2) polymerizing the emulsifier into the polymer so that the emulsifier is permanently bound.
Although commonly used, the first approach results in increased costs due to the additional required operational steps. The second approach is less commonly employed because such emulsifiers typically are very expensive and have limited scope primarily due to low polymerization activity.
In U.S. Pat. Nos. 3,907,870 and 3,980,622 Kozuka et al. disclose polymerizable emulsifying agents having the formula; ##STR1## wherein R is an alkyl group having 6 to 22 carbon atoms, M is H, Li, Na, K, or NH.sub.4, and n is an integer of 2 to 4, inclusive.
In commonly assigned U.S. Pat. No. 4,544,726, Alford, a co-inventor in this application, disclosed half vinyl esters of C.sub.21 -dicarboxylic acid as emulsion polymerization emulsifiers. Although excellent emulsifiers, the half vinyl esters exhibited some stability problems under certain conditions.
An object of the present invention, therefore, is to provide a novel latex composition from which, upon polymerization, the emulsion polymerization emulsifier will not migrate. A further object of this invention is to provide a polymerizable emulsifier for use in an emulsion polymerization process which, upon polymerization, will be permanently bound. Also, an object of this invention is to provide an emulsifier which neither inhibits polymerization nor adversely affects the polymer physical properties.